Windows are sometimes coated with transparent metallic coatings that can be used to electrically heat the window. By applying a DC voltage to the metallic coating, electric current is caused to flow through the coating and across the surface of the window thereby heating the window. Such embodiments are typically used to defrost or defog the window.
One of the consequences of using metallic coated windows is that they can attenuate the propagation of RF signals through the window. As a result, wireless communication into and out of buildings, vehicles, and other structures that use metallic coated windows to defrost or defog the window can be restricted. One solution for applications in which the metallic coating interferes with the propagation of signals through the window has been to remove a portion of the metallic coating that interferes with the signals. Removal of the coating facilitates the transmission of RF signals through the portion of the window where the coating is removed. However, removal of the metallic coating also negates heating of the uncoated area allowing ice and snow to accumulate and remain on the uncoated region. In addition, it has been found that such uncoated areas resulted in localized high current or low current regions that create hot spots and cold spots in and around the uncoated area. Such thermal disparities result in difficulties with defrosting and deicing capabilities and also can cause localized curvatures of the glass that results in optical distortions.
One manner of addressing such problems and limitations related to metallic coatings has been to create a frequency selective surface (“FSS”) area in the coating. A FSS is a periodic pattern in the heatable coating having either one or two dimensions (i.e. single or double periodic structures) that acts as a filter for one or more bandwidths. Depending on the physical construction, materials and geometry of such structures, they are categorized as low-pass, high-pass, band-pass and band-stop filters. For example, as applied to automobile windshields, the FSS acts as a band-pass filter for RF communication signals.
Several FSS designs have been proposed for which laser deletion is used to remove portions of the metallic coatings and generate an FSS area in the coatings that allows RF signals to pass through with limited attenuation. U.S. Pat. No. 6,356,236 B1 to Maeuser illustrates a low loss 1.5 mm FSS grid pattern with line width of 0.3 mm. This FSS grid can be used for an electronic toll collection (ETC) system that is operated at 5.8 GHz. However, the FSS grid pattern creates a break in the electrical current path that blocks even heating across the window. Therefore, such a grid pattern is not preferred for a heatable windshield application. U.S. Pat. No. 5,867,129 to Sauer discloses FSS patterns that incorporate vertical slots or cross-shaped slots with a length that is appropriately tuned for 5.8 GHz applications. However, those designs have not met performance requirements due to high losses.
For heated window applications, U.S. Pat. No. 6,860,081 B2 to Walton and U.S. Pat. No. 7,190,326 B2 to Voeltzel illustrate a variety of different vertical FSS patterns that allow DC current to pass through the FSS window for heating purposes. The slots are oriented vertically so that only horizontally polarized signals can pass the FSS window with low loss. Vehicle electronics of different applications operate at various frequency bands. Such electronics require good RF transmittance in all applicable bands and for all applicable polarizations, including vertical and circular polarizations as well as horizontal polarizations. The designs described above fail to adequately meet those requirements. U.S. Pat. No. 8,022,333 B2 to Maeuser discloses an FSS design on the third visor area of a windshield with low loss conductive material to improve performance. That design requires cover by black paint so the only feasible location for the electronic device is the third visor area of the windshield. Because rain sensor, IR camera, and night vision camera devices already are normally located on the third visor area, space limitations and possible EMC issues make the addition of still more devices in that area somewhat impracticable.
As the demand for vehicle electronics continues to rapidly grow, an increasing number of antennas also have been integrated in the vehicle. For instance, AM/FM radios, TV, cellular phone, remote keyless entry, global positioning system, electronic toll collection, and radar systems are all included in many vehicles. Accordingly, there was a need in the prior art for a metallic coated window having an FSS that would permit the transmission of RF signals. Furthermore, there also was a need for facilitating RF transmission through a window while still enabling electric current flow across the panel in a manner that does not create localized hot spots and cold spots around the perimeter of the FSS area.